This invention relates to a process for measuring a cloud point of polyglycerol-fatty acid ester, a process for evaluating properties of the polyglycerol-fatty acid ester based on the results of the cloud point measurement, and a process for producing the polyglycerol-fatty acid ester using the above-mentioned processes.
Polyglycerol-fatty acid esters (hereinafter referred to as "PGE") are widely utilized as surfactants or emulsifying agents for foods, medicines, cosmetics, or the like. A ratio of a hydrophilic moiety to a lipophilic moiety in the PGE, namely a hydrophilic-lipophilic balance (hereinafter referred to as "HLB") is one of important factors for determining a surface activity of the PGE, when the PGE is mainly used as surfactants. The HLB of the PGE is adjusted by controlling the degree of polymerization of polyglycerol (hereinafter referred to as "PG") or selecting a suitable feed ratio of raw materials for controlling an amount of fatty acid ester (average esterification degree) bonded to a hydroxyl group of the PG.
The PGE is mainly produced by esterifying the PG obtained by the polycondesation of glycerol with fatty acid or derivatives thereof in the presence of an alkaline catalyst. The esterified product (PGE) is used as surfactants or the like, directly after the production or after simple purification treatments in which coloring or odor components are removed from the product, for example, by stream distillation.
In order to produce the PGE having a required HLB and average esterification degree, the production conditions are adequately selected in the following manner, that is, PG and fatty acid fed at a particular ratio are first reacted to obtain the PGE. The resultant PGE is then analyzed to determine the esterification degree of the PGE and to simultaneously ascertain whether the PGE exhibits properties such as a surface activity similar to those of an aimed PGE. Accordingly, if the PGE produced differs from the aimed PGE, a number of experiments or tests must be repeated to optimize the reaction conditions including the feed ratio of the fatty acid to PG, the reaction time, the reaction temperature, the amount of the catalyst used, or the like and it takes much time to determine the production condition.
Conventionally, there have been used various chemical analyzing methods for determining properties of the PGE. For example, in order to evaluate the esterification degree and the amount of residual fatty acid, the measurements of acid value, ester value and hydroxyl number of PGE has been used. Further, various evaluation methods such as an analysis of ignition residues has been used to determine amounts of soap and residual catalyst.
However, the PG skeleton which is hydrophilic moiety of PGE is a polycondensate of glycerol and therefore exhibits a particular distribution of degree of polymerization thereof. In addition, the PGE contains not only linear polymers but also branched or cyclic polymers. Since the viscosity of PG generally increases so that the degree of polymerization thereof becomes larger, it becomes extremely difficult to purify the PG by separating the cyclic polymers or the like from the polycondensate. As a result, the PGE which is a reaction product obtained by esterifying hydroxyl groups of the PG with fatty acid, is a mixture composed of the PGEs having different PG skeletons and various esterification degrees, and unreacted PG.
Also, the PGE as the esterified product contains soaps as a by-product produced by the reaction of the alkaline catalyst with raw fatty acid, or unreacted fatty acid which is remained in case where the insufficiently esterification reaction of the PG and the fatty acid is carried out or in case where fatty acids are used in an excess amount relative to a stoichiometric amount thereof.
Thus, since the PGE produced comprises a mixture having a complicated composition, even though the PGEs exhibit similar or same esterification degrees, respectively, the properties such as emulsifying stability of the PGEs may be quite different from each other. As a result, by conventional chemical analyzing methods used for determining average esterification degrees and the amount of unreacted PG, the properties of the PGE produced cannot be evaluated sufficiently.
On the other hand, there have been used another type of evaluation methods for surfactants, by using the properties of the surfactants measured directly. For example, there is known a method for measuring a cloud point of an aqueous solution of a polyoxyethylene-based nonionic surfactant produced from ethylene oxide ("Dictionary of Technical Terms for Fats and Oils" (Saiwai Shobo)). Generally, the "cloud point" in surfactants is determined as a temperature at which an aqueous solution of the nonionic surfactant is separated into two phases with a rise of temperature. Such separated two phases can return to homogeneous phase with a decline of temperature.
In the polyoxyethylene-based surfactants, there is known a correlation in the range of the cloud point and an average degree of polymerization of the polyoxyethylene chain. The cloud point of the polyoxyethylene-based surfactant is increased depending on the increase of average degree of polymerization of the polyoxyethylene. Hence, in order to produce an aimed surfactant, the reaction conditions is adjusted such that the degree of addition polymerization of ethylene oxide is present in an adequate range. Such an adjustment of the reaction conditions is achievable only in the case where a hydrophilicity of an ether group-existing in polyoxyethylene of the surfactant become poor with a rise of temperature.
On the other hand, in the case of the PGE, since a hydrophilic moiety thereof is a polycondensate of glycerol and is different from the polyoxyethylene chain of the above-mentioned surfactant, not only a hydroxyl group and an ether group coexist in the PGE but also the PGE has an extremely complicated composition caused by a branched-chain condensation and cyclic polycondensation due to the existence of the hydroxyl group.
As a result of earnest studies by the present inventors on evaluation process of properties of the PGE, it has been found that under a particular condition, the PGE has a cloud point similar to the known cloud point of the polyoxyethylene-based surfactant, and the cloud point of the PGE can be measured at a temperature of 0.degree. to 100.degree. C. by preparing a homogeneous aqueous PGE solution thereof to which a salt and/or polyhydric alcohol are added a PGE solution. On the basis of these findings, the present invention has been attained.